MB High Pressure Mercury Vapour

Updated06-V-2016

Mercury Vapour lamps operating at high pressure (but below 100 watts per centimetre of arc length) and with arc tubes of quartz glass are classified as the MB style. Mercury vapour pressure is typically around ten atmospheres. The reason for the creation of this second category of mercury lamp was to allow the efficacy to be maintained as high as possible for low wattage sources.

The luminous efficacy of any discharge lamp falls as wattage is reduced. It was not feasible to make an MA style lamp below about 150W because its efficacy dropped so much that it could no longer compete with filament lamps. However if the mercury vapour pressure and power loading is increased, luminous efficacy also rises. This was not possible with MA lamps because the arc tube would melt at the higher temperatures which are present at higher power loading.

Quartz glass having a very high softening temperature was necessary to make the low wattage mercury lamp feasible. Quartz is not an easy material to use though. Its coefficient of thermal expansion is so low that no metal wire can be sealed through it directly. One solution came from the work of Denis Gabor at German Osram and later BTH, who invented the molybdenum foil seal. Moly foil of very thin cross-section expands so little that the quartz barely knows it's there. The edges of the foil are feathered, either by rolling or etching, and these knife edges can deform and bury themselves in the quartz as they expand, without cracking it. An alternative seal based on a stack of glasses having graded expansion coefficients was developed by Cornelis Bol at Philips, but was ultimately superseded by the foil seal.

Following the development of these seals, the first high pressure quartz mercury lamps were marketed in 1936 by Philips. The higher wattage MA lamps continued to be made with aluminosilicate arc tubes until the late 1950's on account of the large price penalty that quartz carried with it in the early days, and that an increase in their power loading did not bring such a large advantage as for the low wattages. Eventually though once the price of quartz had fallen sufficiently, and means had been developed for mechanising the seal production, the MB lamp completely superseded the former MA range.